The present invention relates to a magnetic detecting method and a magnetic detector, in which magnetizers applys magnetic fields crossing over an object made of magnetic material, such as a steel plate, and magnetic sensors are used to detect leakage fluxes generated at magnetically defective portions of the object.
A magnetic detector utilizes magnetism to detect magnetically defective portions of an object, e.g., a thin steel strip, such as flaws or inclusions existing in the surface of the thin steel strip. It is reported that a magnetic detector having a group of magnetic sensors arranged linearly for detecting magnetic fluxes can continuously detect defects existing in a moving thin steel strip, over the entire width thereof. (See Published Unexamined Japanese Utility Model Application 63-107849.)
FIGS. 46A and 46B are sectional views schematically showing the magnetic detector for continuously detecting defects in a moving thin steel strip. FIG. 46C is a side view showing the magnetic detector and also a support device supporting the detector.
As is illustrated in FIG. 46C, a horizontal arm 12 is supported in a frame 11 secured on the floor of a building, by means of a pair of spring members 13a and 13b. The horizontal arm 12 can thereby move up and down. The magnetic detector has a hollow roll 1 and a rigid shaft 2. The rigid shaft 2 is fastened to the center part of the horizontal arm. Two guide rolls 14a and 14b are located at the sides the frame 11, respectively, for guiding a thin steel strip to the outer circumferential surface of the hollow roll 1 of the magnetic detector.
As FIGS. 46A and 46B show, one end of the rigid shaft 2 extends through the hollow roll 1 made of non-magnetic material, located coaxial with the hollow roll 1. The other end of the rigid shaft 2 is fastened to the horizontal arm 12. The rigid shaft 2 is supported and located coaxial with the hollow roll 1, by means of a pair of rolling bearings 3a and 3b. Hence, the hollow roll 1 can freely rotate around the rigid shaft 2.
In the hollow roll 1, a magnetizing core 4c, substantially U-shaped, is fastened to the rigid shaft 2 by a support member 5, having its magnetic poles 4a and 4b positioned near the inner circumferential surface of the hollow roll 1. A magnetizing coil 6 is wound around the magnetizing core 4c. The core 4c and the coil 6 constitute a magnetizer 4. A plurality of magnetic sensors 7a, forming a group 7, are fixed on the rigid shaft 2, located between the magnetic poles 4a and 4b and arranged parallel to the axis of the shaft 2.
A power-supply cable 8 for supplying an exciting current to the magnetizing coil 6, and a signal cable 9 for supplying signals output by the magnetic sensors 7a pass through the rigid shaft 2 and extend outwardly from the shaft 2. Hence, the magnetizer 4 and the group 7 of magnetic sensors are fixed in place, whereas the hollow roll 1 can rotate around the magnetizer 4 and the sensor group 7, slightly spaced apart therefrom.
When the hollow roll 1 of the magnetic detector described above is so moved as to have its outer circumferential surface pushed, with a predetermined pressure, onto one side of a thin steel strip 10 moving in the direction of arrow a, the hollow roll 1 is rotated in the direction of arrow b. This is because the shaft 2 is fastened to the horizontal arm 12.
In the magnetic detector, when an exciting current is supplied to the magnetizing coil 6, the magnetic poles 4a and 4b of the magnetizing core 4c and the thin steel strip 10 form a closed magnetic path. If defects exist in the steel strip 10 or in the surface thereof, the magnetic resistance in the strip 10 changes, generating leakage fluxes. The leakage fluxes are detected by those of the magnetic sensors 7a forming the group 7, which oppose the defective portion of the strip. These magnetic sensors 7a output signals representing the defects.
The levels of the signals detected correspond to the sizes of the defects existing in the thin steel strip 10 or in the surface thereof. The sizes of the defects and the positions they assume widthwise of the strip 10 can be determined by measuring the levels of the output signals.
In this magnetic detector, each magnetic sensor 7a is located at a midpoint between the magnetic poles 4a and 4b. The reason for this specific positioning will be described.
FIG. 47 is a schematic representation of the main section of the magnetic detector shown in FIGS. 46A and 46B. In the detector, the magnetic poles 4a and 4b are arranged, opposing a thin steel strip 10 which has no defects and which is not moving. The magnetizing coil 6 is excited with a DC current. A magnetic field is thereby generated in the vicinity of the magnetic poles 4a and 4b. This magnetic field has a vertical magnetic-field distribution D shaped like a sine wave and a horizontal magnetic-field distribution F shaped like an angle i.e. a curve having substantially symmetrical rising and falling slopes. In the magnetic-field distribution D, the magnetism is maximum and minimum at the poles 4a and 4b, respectively, as is shown in the figure. In the distribution F, the magnetism is maximum at the midpoint between the magnetic poles 4a and 4b. Hence, a magnetic sensor of vertical type, which responds to magnetism applied in the vertical direction, will not be influenced by the magnetic field if the sensor is located at the midpoint between the poles 4a and 4b spaced by distance W, at which the curve of the vertical distribution D crosses the zero-level line.
If a magnetic sensor of horizontal type, which responds to magnetism applied in the horizontal direction, is located at the midpoint between the poles 4a and 4b spaced by distance W, and if the signal output by the horizontal-type magnetic sensor is differentiated, the output signal will have a waveform which is similar to the vertical magnetic-field distribution D in the vicinity of the midpoint between the magnetic poles 4a and 4b. The waveform of the differentiated signal crosses the zero-level line at the midpoint between the poles 4a and 4b. Thus, like the vertical-type magnetic sensor, the horizontal-type magnetic sensor will not be influenced by the magnetic field.
It is, however, desirable that a vertical-type magnetic sensor be used, rather than a horizontal-type one. This is because it can generally be said that a horizontal-type magnetic sensor needs to have a broad dynamic range since, as indicated above, the floating flux at a non-defective portion of a base metal exhibits a great magnitude. To obtain the same output as this, by means of a horizontal-type magnetic sensor, the output of the horizontal-type magnetic sensor must be first processed by a differentiation circuit. Consequently, the detector will be complex. Further, the ratio (f.sub.S /f.sub.N).sub.H of the frequency f.sub.S of a signal component resulting from a magnetically defective portion to the frequency f.sub. N of a noise component achieved by the use of a horizontal-type magnetic sensor is less than the ratio (f.sub.S /f.sub.N).sub.V achieved by the use of a vertical-type magnetic sensor.
Hence, when a vertical-type magnetic sensor is used, it will be easier to remove noise from the output signal. It is therefore better to use a vertical-type magnetic sensor for the purpose of simplifying the detector for practical use. It is not to say that a magnetic sensor other than a vertical-type one cannot detect a magnetically defective portion.
Even if the magnetic sensors 7a are not located at the midpoint between the magnetic poles 4a and 4b as is shown in FIG. 48, it is possible to eliminate such influence of a magnetic field as has been explained above. Namely, the voltages Vov and Voh which each magnetic sensor 7a so located outputs are determined beforehand from the magnetic-field distributions D and F. Then, as is shown in FIG. 49, a fixed bias voltage output by a bias voltage generator 16 is adjusted to the voltage Vov or Voh, and the voltage Vov (Voh) is subtracted from the output signal of the magnetic sensor 7a by means of a subtracter 15.
Unless otherwise noted, the following description relates to the case where vertical-type magnetic sensors are positioned at the midpoint between the magnetic poles.
The vertical magnetic-field distribution D illustrated in FIG. 47 is the one observed when a flawless thin steel strip 10, not moving, opposes the magnetic poles 4a and 4b. In practical use of the magnetic detector, however, the thin steel strip 10 is moving at speed V in one direction. As the strip 10 moves so, it is magnetized by the magnetic poles 4a and 4b. Because of the speed effect of the strip 10, i.e., an object, moving through the excited magnetic field, the flux distribution is biased in the direction in which the object is moving. More specifically, as the object, which is a conductor, moves through the magnetic field, an eddy current flows in the object, which generates a magnetic field. This magnetic field is assumed to bias the flux distribution in said manner.
As a result, the curve of the vertical magnetic-field distribution crosses the zero-line, not always at the midpoint between the magnetic poles. The vertical magnetic-field distribution shifts in parallel, in the direction in which the object is moving, as is indicated by a vertical magnetic-field distribution E illustrated in FIG. 47.
Thus, while the thin steel strip 10 is moving, the curve representing the vertical magnetic-field distribution E does not cross the zero-level line at the midpoint (X=0) between the magnetic poles 4a and 4b. Therefore, floating fluxes exist at the midpoint.
Floating fluxes are detected around the object, and are distinguished from the leakage fluxes generated due to surface defects, internal defects, and magnetically defective portions such as welded portions. The floating fluxes outwardly emanate from mostly the object, i.e., a bulk, or the magnetizing core of the magnetizer. The floating fluxes, therefore, have a distribution which is similar to each of the magnetic-field distributions illustrated in FIG. 47.
Naturally, the floating fluxes change as the moving speed V of the thin steel strip 10 increases. The floating fluxes change, too, as the exciting current I supplied to the magnetizer 4 is increased. FIG. 50 is a diagram representing how the output voltage of a vertical-type magnetic sensor 7a located at the midpoint (X=0) actually varied as the moving speed V of a flawless thin steel strip 10 was increased from 0 m/min to 1200 m/min. The characteristics shown in FIG. 50 were recorded as the exciting current I for the magnetizing coil 6 was set at 0.25 A, 0.50 A, and 0.75 A. As can be understood from the diagram, the floating fluxes increase as the moving speed V and the exciting current I are increased.
There is a specific range for the intensity of a magnetic flux which the magnetic sensor 7a can detect. When the sensor detects a magnetic flux having an intensity higher than a predetermined value, it outputs a saturated signal. FIG. 51 is a diagram showing the relation between the moving speed V of the flawless thin steel strip 10 and the relative output voltage of the magnetic sensor 7a, which relation is an actually recorded one. As can be understood from this diagram, too, the output signal generated form the floating flux is saturated when the moving speed V of the strip 10 is about 600 m/min if the exciting current I is 0.2 A.
It is often demanded that each magnetic sensor 7a have high sensitivity to leakage fluxes resulting from defects. For example, the sensor is expected to detect so small a flux as about mm gausses. Each magnetic sensor 7a should therefore have its sensitivity enhanced very much.
FIG. 52 is a diagram illustrating how the output voltage of the magnetic sensor 7a actually changed as magnetic fluxes of various intensities were applied, crossing the sensor which has such sensitivity that it outputs 1 V when the intensity of the flux is 1 gauss. As can be understood from this diagram, as the sensitivity of the sensor 7a is increased, its output voltage will be saturated when a flux of about 6 gauss crosses the sensor.
This phenomenon that the output signal of the magnetic sensor 7a is saturated becomes more prominent when the moving speed V of the thin steel strip 10 increases. FIG. 53 is a diagram representing the relation which the output of the sensor 7a and the exciting current I had when a steel strip 10 having an artificial defect having a diameter of 0.6 mm was put to defect detection.
As this diagram reveals, the output of the magnetic sensor 7a will decrease, rather than be saturated only, when the exciting current I is increased over a certain value, as the moving speed V of the steel strip 10 and the exciting current I for the magnetizer 4 are increased in order to enhance the defect-detecting sensitivity.
Hence, even if the sensitivity of the magnetic sensor 7a is increased, the sensor is still unable to detect a small defect. Further, since the floating flux emanating from a flawless steel strip is far more intense than the leakage flux generated at a small defect existing in a steel strip, the output signal will be saturated due to the floating flux when the detection sensitivity of the magnetic sensor 7a is increased. As a consequence, it is impossible to detect a small defect with high accuracy.
The problem is not a phenomenon specific to only a magnetic detector of the type which has a hollow roll as is disclosed in Published Unexamined Japanese Utility Model Application 63-107849. In particular, a decrease in the defect-detecting accuracy, resulting from a leakage flux, is the phenomenon which is generally observed in so-called magnetic detection technology of detecting magnetically defective portions by means of a magnetizer.
The thin steel strip 10, i.e., the object of magnetic detection, has its magnetizing characteristic changed with the speed V at which it is moved. The higher the moving speed V of the strip 10, the less liable the steel strip 10 is magnetized, because of the speed effect described above. Consequently, any defect of a specific size will be detected to have a different size if the moving speed V of the thin steel strip 10 changes.